JP-A-6-104672 discloses a clamping circuit having the construction shown in FIG. 6. The clamping circuit is equipped with a clamping transistor 201 provided between a terminal 200 and ground to clamp the voltage V appearing at the terminal 200 to a predetermined potential level, a resistor 202 for applying a gate voltage VG to the transistor 201, transistors 203, 204 for supplying constant current Lo to the resistor 202 and a constant current source 205.
The transistor 201 comprises a p-channel MOS transistor in which the source thereof is joined to the substrate and connected to the terminal 200 and the drain thereof is connected to the ground. The resistor 202 is constructed by a trimmable resistor such as a polysilicon resistor or the like, and the resistance value thereof is set to a desired value R. The resistor 202 is connected between the gate of the transistor 201 and ground and applies to the gate of the transistor 201 the constant voltage VG corresponding to the product lo×R of the current lo flowing in the resistor 202 itself and the resistance value R.
A transistor 203 is constructed by a p-channel MOS transistor. The substrate and the source thereof are commonly connected to each other, the source is connected to the terminal 200, the drain thereof is connected to a terminal 210, and the gate thereof is connected to the gate and drain of the transistor 204. The transistor 204 is also constructed by a p-channel MOS transistor. The substrate and the source thereof are commonly connected to each other, the source thereof is connected to the terminal 200, the drain thereof is connected to the gates of the transistors 203 and 204, and also connected to the constant current source 205. The transistors 203 and 204 constitute a current mirror circuit, and the constant current source 205 extracts constant current Lo from the transistor 204, so that the mirror current lo flows.
In the clamping circuit disclosed in JP-A-6-104672, it is needed to adjust the resistance value of the resistor 202 or the current value of the constant current source 205 in order to adjust the clamping voltage at the terminal 200. However, in order to design the resistor 202 so that it is trimmable, it is needed to separately provide the corresponding process. Accordingly, when it is not scheduled from the beginning to carry out such a process, it is difficult to form the trimming resistor.
When the trimming resistor is not usable, the gate potential of the transistor 201 serving as a reference voltage for the operation suffers a specific temperature effect which the constant current source 205 has.